The gravitational sinking of organic debris from ocean ecosystems is a dominant mechanism of the biological carbon pump (BCP) that regulates global climate. The fraction of primary production exported downward, the e-ratio, is an important but poorly constrained BCP metric. In mid- and high-latitude oceans, seasonal and local variations of sinking particle fluxes modulate strongly the e-ratio. These locally-specific e-ratio variations and their ecological foundations are here encapsulated in the term ‘export systems’ (ES). ES have been partly characterized for a few ocean locations, but remain largely ignored over most of ocean's surface. Here, in a fully conceptual approach and with the primary aim to understand rather than to estimate ocean carbon export, we combine biogeochemical (BGC) modeling with satellite observations to map ES at fine spatio-temporal scales. We identify four plausible ES with distinct e-ratio seasonalities across mid- and high-latitude oceans. The ES map confirms the outlines of traditional BGC provinces, and unveils new boundaries indicating where (and how) the annual relationship between carbon export and production changes markedly. At six sites where ES features can be partially inferred from in situ data, we test our approach and propose key ecological processes driving carbon export. In the light of our findings, a re-examination of 1,841 field-based e-ratios could challenge the conventional wisdom that e-ratios change strongly with latitude, suggesting a possible seasonal artefact caused by the timing of observations. By deciphering carbon export mechanistically, our conceptual ES map gives timely directions to emergent ocean robotic explorations of the BCP.

Key Points

A plausible mechanistic map of ocean carbon export is derived from spatio-temporal changes of the e-ratio in mid and high-latitude oceans

The map unveils the possible distribution and boundaries of four main systems of export and explains their potential ecological drivers

The debated view that e-ratios vary mostly with latitude is challenged by results suggesting instead an effect of e-ratio seasonality

Plain Language Summary

The oceanic biological carbon pump regulates Earth's climate by carrying part of the dissolved CO2 fixed by surface planktonic ecosystems to deeper depths. Controls that ecosystems exert on the fraction of carbon exported, the ‘e-ratio’, have been partly identified at a few ocean locations, but insufficiently to achieve a global mechanistic understanding especially where processes are affected by seasonality. Using a fully conceptual approach combining biogeochemical modeling and satellite observations, we map plausible e-ratio variations at an unprecedented resolution in mid- and high-latitude oceans. The map unveils the possible distribution and boundaries of four main systems of export and investigate their ecological drivers. Our work highlights long-standing inconsistencies from field-based e-ratios and provides a timely road map for emergent ocean robotic explorations.